How do I model a transfusion blood bag in Blender?
Here are some tips for modeling a very simple blood bag.

1) Use Subdivision modifier to get that smooth shape.
2) Use Mirror modifier for perfect symmetry.
3) Keep quads everywhere, so you can use loop cuts  



1) Inner and outer shape
- start with a simple plane and create the inner shape
- add loopcuts, move vertices.
- use Subdivision modifier to get the smooth shape.
- Use Mirror modifier for perfect symmetry and to save time.
  


use the same approach to create the outer shape  
make sure it connects nicely to the inner shape and there are no n-gons  





position some loopcuts for the tube openings.  




2) Add some thickness
We need some thickness to create tube openings.
You can add some loopcuts inside the edge to better support the geometry.


3) Create tube openings  (you can apply the Mirror modifier before that)
- add a circle and prepare an empty the loop on your bodybag (by removing the faces inside)
- connect through Edge > Bridge Edge loops.
- make sure that the number of vertices is the same. (8 in my case)
- If you want more vertices in your circle, just add more loopcuts.



Add some loopcuts for sharper edges.


Here is the file:


And the reference image I used: